There are two types of organic EL display devices, a passive (a simple matrix) and an active (an active matrix), and development of both is being enthusiastically performed. Furthermore, organic materials are divided into low molecular weight (monomer) organic EL materials and high molecular weight (polymer) organic EL materials. Both are being vigorously researched, where a film of low molecular weight organic EL material is mainly formed by evaporation, while a film of high polymer organic EL material is mainly formed by application.
A drawback with organic EL devices is that they are difficult to drive using simple two-terminal schemes because of their lack of memory. The rise and decay time of an organic EL device is very fast and it does not have intrinsic memory. To overcome this problem, thin-film-transistor (TFT) circuits have been developed to drive organic EL devices. As illustrated in FIG. 1, these circuits include four or more TFTs, a storage capacitor and an organic EL pad arranged on a substrate. The storage capacitor enables the excitation power to an addressed EL element to stay on once it is selected.
To obtain uniform images on TFT active matrix OLED/PLED displays, it is necessary to ensure uniformity in the brightness of each pixel. This brightness is dependent on the current in the OLED/PLED device that is driven by each pixel circuit. An important issue for TFT active matrix OLED/PLED displays has been the pixel driving technology needed to achieve uniformity in OLED/PLED currents. The main difficulty is non-uniformity in the current/voltage characteristics of the TFTs that form part of the pixel circuit.
The capacitor current memory method achieves a “current memory” that supplies continuous forward current to the OLED/PLED device. However, obviously the memory requirements of a TFT active matrix OLED/PLED display cannot be achieved using a “current memory”, as these do not exist for this circuit, but must be achieved by storing a voltage on a capacitor that represents the required circuit.
However, voltage programmed circuits suffer from non-uniformity issues. In LTPS-TFT technology, the variations of device characteristics (mainly on threshold voltage) caused by device aging or manufacturing process are unavoidable. Simple pixel driver schemes with two transistors per pixel are sensitive to threshold voltage variations. Schemes with four transistors per pixel have therefore been proposed. The multi-transistor pixel circuit structure is capable of an improved adjustment of the variation of threshold voltage and becomes a better method for active matrix OLED/PLED displays. Self-compensating current method compensates not only threshold voltage but also mobility distribution.
Some of the non-uniformity issues are caused by lack of dynamic range limiting signal swing at the input. When the input is low the threshold voltage of the drive transistor affects low gray scale levels. Switching effects including clock feedthrough and charge injection affect the storage capacitor causing output current error. Second order crosstalk affects display uniformity. Another important display issue is registration of multitask patterns on plastic substrates.
While successfully overcoming the above-mentioned problem, new problems in manufacturing are created. The storage capacitor process and deposition are very complicated and difficult to achieve in a fabrication process. The TFTs fabrication requires several mask steps whose difficulty and cost increase dynamically as the display size increases. Plus if the substrate is plastic an expensive laser annealing process is used in fabrication of the TFT.
Another problem when using TFT circuits is almost all have no capability for reverse bias of the OLED element. The result of this is metal migration into the organic layers is not prevented. For example metal migration from the ITO front electrode into the organic layers.